Ytterbium-doped yttria (Yb:Y2O3) is an excellent laser host material due to its hardness, strength, and transparency in the range of 0.4 to 10 μm. Its thermal properties enable it to operate at higher power and dissipate heat generated during laser operation better than other laser hosts such as yttrium aluminum garnet (YAG). However, single crystal Yb:Y2O3 is difficult to produce in large sizes and the necessary configurations for high power lasers.
Polycrystalline Yb:Y2O3 can also achieve this high performance if it is produced in a manner as to produce a fine grained material with clean grain boundaries, very low porosity, and less than 10 ppm levels of impurities.
A transparent polycrystalline Yb:Y2O3 laser material can be manufactured by densifying Yb:Y2O3 powder. However, attempts to fabricate transparent ceramics with traditional undoped Y2O3 powders have been unsuccessful due to the large size of the particles, the high impurity level, and the presence of hard agglomerates in the powder, which can lead to high scattering and absorption losses in the final material
Various processes, including decomposition of the salts, co-precipitation, hydrothermal synthesis, spray drying, and sol-gel synthesis, have been reported to produce fine Y2O3 powders. See L. R. Furlong et al., “Sintering of Yttrium Oxide,” Ceram. Bull., 45, 1051, (1966); R. Subramanian, et al, “Synthesis of nanocrystalline yttria by sol-gel method,” Mater. Let., 48, 342 (2001); H. Tomaszewski, et al., “Crystallization of yttria under hydrothermal conditions,” J. Eur. Ceram. Soc., 17, 403, (1997); T. Hours, et al., “Preparation and characterization of yttrium oxide by a sol-gel process,” Am. Ceram. Soc. Bull., 71, 200, 1992).
Among these processes, the co-precipitation of the precursors using inorganic salts in a base condition and a subsequent calcination is the most convenient and cost effective technique, and is suitable for mass production of homogeneous powders. However, the powder produced by this process includes large, hard agglomerates, and attempts to fabricate transparent ceramics using these powders always result in an opaque or semi-transparent product containing large grain size and numerous structural and morphological defects.
Hard agglomeration is believed to be caused by the strong intra- and/or inter-molecular hydrogen bonding between hydroxide or hydroxynitrate precursors and water molecules. Small and extremely polar water molecules attract the precursors to pack close together upon drying and cause the precursor molecules to agglomerate together during the drying process. Once the powder is agglomerated, it is almost impossible to break it into smaller particles or soft agglomerates. Such agglomerated powders become even harder after calcination and it makes subsequent processing very complicated and troublesome. Although such powders can be broken down into smaller particles by various milling process, even after milling, they still contain hard agglomerates having a size as large as about 10 microns. In addition, using a milling step can be problematic, especially for the production of transparent ceramic laser materials where the transparency and the lasing property is affected by even ppm level of metal impurities as well as morphological and structural defects in the ceramic, since the powder can be contaminated during the process.
Thus, to maximize the optical transparency of the ceramic and to reduce scattering, it is desirable to obtain high purity nano-sized powders without hard agglomerates by preventing hard agglomerates before they start to form.